This invention relates generally to an arrangement for transforming a rotary movement of a smooth shaft into a thrust movement of a nut and more specifically, the invention relates to a roller nut for effecting such transformation. The roller nut employed in the arrangement according to this invention is of the type where several roller cages having annular configuration surround the shaft at different angles to each other and are held together in a common housing whereby center passages of the roller cages are in engagement with the smooth surface of the shaft.
In the construction of machines and apparatuses, screws are frequently needed that should be capable of providing thrust movements over long distances. The machining of threads for the long screws of this type brings along several difficulties, especially when the screws are supposed to have a high degree of accuracy so that neither play nor wear between the bolt and nut take place. It is also frequently required that the bolt and nut arrangement of this kind have a smooth cooperation with minimum friction.
Screw or bolt nuts have been known which employ rolling elements arranged for rolling in correspondingly formed grooves in the bolt and/or in the nut, thus avoiding any sliding movement and thereby also the wear. As an example of this known type are circulating on cyclic bolt nuts in which the bolts upon completion of one or more turns of the thread are guided to return to the beginning of the thread. Such screw drives however are hard to manufacture and consequently very expensive so that in many cases they cannot be used.
Known are also frictional screws consisting of one or more rings which are pressed by resilient forces against the smooth surface of the shaft in such a manner as to be able to perform a screw-like motion when the planes of the rings and of the shaft intersect each other at the same angle.
For example, in the German Pat. No. 1,057,411 a roller ring drive is described in which the aforementioned angles are adjustable so that the screw motion has an adjustable pitch. Nevertheless, technological expenses in making this embodiment of drives having minimum sliding friction are too high even if a screw motion having a single fixed pitch only is to be designed. Moreover, the drives having an adjustable pitch angle have the disadvantage that the shaft is displaced laterally from the center of the ring if transverse forces act against the ring and no additional guide between the ring housing and the shaft is provided. It is true that the rings lying upon the shaft are in balance due to their contact pressure but they do not have a stable transverse position since the almost punctiform line or area of their rolling contact lies on the swing axis of the rings. Consequently, additional lateral guides are necessary in which, however, sliding motions with frictional losses will result and also the wear. In roller ring drives having three rings the center ring is twice as much loaded as the two outer rings (FIG. 3 of the German Patent No. 1, 057 411). The maximum rolling capacity is thereby determined by the allowable specific surface pressure in the point or line of rolling and by the bearing strength or capacity of the inner ring. Since, as mentioned above, the prior art rings bear against the shaft almost at one point only, the contact pressure forces are distributed on the ring supporting balls more irregularly than in the case of a simple support of the shaft where the shaft occupies the entire inner ring. For this reason the bearing capacity or the strength of the bearing cannot be fully utilized.